1. Field of the Invention
The present invention relates to the field of the prevention against explosion of on load tap changers of electrical transformers.
2. Description of the Relevant Art
Due to their structure, on load tap changers (OLTC) exhibits small electrical arcs during commutations from one step to an other. The OLTC are generally at least in part filled by a dielectric fluid such as oil. The oil used can ignite above a temperature of the order of 140° C. The electrical arcs increase the oil degradation inside the OLTC. The OLTC exhibit losses, for which reason the heat produced needs to be dissipated. This heat dissipation is also made by the oil.
An electrical insulation fault, between phases or between one phase and the ground, firstly generates a strong electric arc which causes actuation of the electrical protection systems which triggers the supply cell of the transformer (circuit breaker). The electric arc also causes resultant dissipation of energy, which causes a gas production, specially hydrogen and acetylene, by dielectric oil decomposition.
After the gas discharge, the pressure inside the OLTC tank increases very rapidly, leading to an often very violent deflagration. The deflagration causes significant tearing of the mechanical linkages of the tank (bolts, welds) placing the gases in contact with the oxygen in ambient air. Since acetylene self-ignites in the presence of oxygen, a fire breaks out immediately and spreads to other items of equipments on the site which are also likely to contain large quantity of combustible substances.
Explosions are caused by insulation ruptures due to short-circuits caused by overloads, voltage surges, gradual deterioration of the insulation, insufficient oil level, the presence of water or mold or failure of an insulation component.
In the prior art, fire extinguishing systems for electric transformers were activated by fire detectors. However these systems operated with a significant lag, when the transformer oil was already burning. The fire outbreak is merely restricted to the equipment concerned to not spreading the fire to the neighboring installations.
A prevention device against explosion and fires of electrical transformers and on load tap changers is known since the late 90's.
The invention improves the situation.